Method for the production of rotors for screw-type compressors

ABSTRACT

The invention is directed to a method for the production of rotors (10) for screw-type compressors (16) and to rotors (10) to be produced according to said method. To allow the use of a straightforward manufacturing method for the production of rotors (10) of a complex geometry, a method is proposed wherein, during the production of the negative mold, material is removed from a negative mold blank to generate correction regions of a rotor (10).

BACKGROUND OF THE INVENTION

The invention is directed to a method for the production of rotors forscrew-type compressors, and rotors to be produced by said method.

For the production of rotors for a screw-type compressor, use ismade--besides machining production techniques--of master mold techniqueswherein the rotor is produced by filling a negative mold with a suitablerotor material. The terms "master mold techniques" or "master molding"are used to define a method wherein a first mold is produced from loosematerial by generating the required cohesion of material (primaryshaping). An example of a master mold technique is a casting processwherein a master mold is filled with material. A master mold techniquespecially adapted to the production of rotors is known from DE 40 35 534A1. According to this method, rotors for screw-type compressors areproduced from fiber-reinforced synthetic material by stacking disks ofsuch fiber-reinforced synthetic material upon each other in the hollowcavity of a negative mold and connecting them to each other byapplication of heat and pressure.

Also the negative mold is generated by a master mold technique wherein arepresentation is produced of the contour of a master rotor, the masterrotor corresponding in shape to the rotor to be produced.

Due to the double use of a master mold technique, the shape of the rotorto be inserted into the screw-type compressor is determined by themaster rotor. The master rotor is usually produced by a machiningtechnique. However, the tools used for producing the master rotor, e.g.slotting or grinding machines, which generate the three-dimensionallycurved surface of the teeth, require cutout portions for the applicationof the tools. For forming the tooth-root regions, it is up to nowrequired that rounded deepened portions are worked by use of a tool intocylindrical outer surfaces extending concentrically to the rotor axis.These rounded deepened portions are necessary to allow the applicationof the tool used for working on the tooth flanks.

When a rotor comprising such rounded deepened portions is to be insertedinto a screw-type compressor together with a second rotor, the secondrotor must have the tips of its teeth provided with edges which engagethe rounded deepened portions during meshing. On the other hand, it isnot possible to effect a sufficient sealing between the rotors. However,the edges of the second rotor enlarge its diameter and thus the diameterof the housing portion enclosing the second rotor. The enlargement ofthe diameter of said housing portion and the rounded portions of theedges cause a deterioration of the sealing properties in the engagementregion of the two rotors. Before the teeth of the rotors during therolling movement in the respective cross section come into mutualabutment, a so-called blow-hole lies therebetween, allowing compressedgases flowing back therethrough to the low pressure side.

Thus, the rotor geometry due to the manufacturing conditions will resultin backflow losses which cause low efficiency in known screw-typecompressors.

SUMMARY OF THE INVENTION

It is an object of the invention to produce rotors of a complex geometryby use of a straightforward manufacturing method.

For providing a screw-type compressor having an increased efficiency, itis required to optimize the rotors with respect to their flow andsealing behavior. This, however, can be accomplished only if such anoptimized geometry of the rotors can also be manufactured in anindustrial production process. The solution of the object of theinvention is based, on the one hand, on a concept for an improvedgeometry for the rotors and, on the other hand, on the provision of amanufacturing method which allows for mass production of such rotors. Inthis regard, the manufacturing method and the corresponding rotors aretwo different aspects of the design concept forming basis of theinvention.

The rotors produced by use of a master mold have a geometry which isdetermined by the geometry of the master mold. The geometry of themaster mold is directly reproduced in the rotors manufactured by use ofthe master mold. Therefore, by changing the geometry of the negativemold, the geometry of the resultant rotor can be determined. If, forinstance, it is detected that a rotor exhibits an unfavorable flowbehavior and if it is then found out in what manner the rotor geometryhas to be changed for improving the flow behavior, changes can be madein the geometry of the negative mold to optimize the geometry of therotor. Thus, by performing a corrective treatment of the negative mold,rotors can be produced to have a corrected rotor geometry as compared toearlier production series.

The flow and sealing behavior is determined particularly by theconfiguration of the tooth-root regions of the winding teeth of theprimary rotor. The secondary rotor meshing with the primary rotor musthave corresponding cylindrical outer surfaces formed on the tips of itsteeth. Due to the novel geometry of the primary rotor, there can be useda secondary rotor having a small diameter and comprising no roundedportions on the tips of its teeth. This provision makes it possible,first, that the housing portion surrounding the secondary rotor canenclose the engagement region of the two rotors still more tightly and,second, that during the meshing movement of the two rotors the blow-holeis closed faster when the teeth come into mutual engagement. Thus, bythe novel geometries of the primary rotor and the secondary rotor, theblow-hole delimited by the housing and the rotors is reduced both underthe aspects of time and space. Both aspects lead to a reduction of thebackflow losses. By producing rotors using a master mold technique, itbecomes possible to reproduce the complex geometries with high precisionand relatively low expenditure.

A further advantage resides in the improved sealing between the tips ofthe teeth of the secondary rotor and the housing portion surrounding thesecondary rotor. The cylindrical outer surfaces of the tips of the teethprovide a better sealing effect than the rounded edges according to thestate of the art.

According to the invention, a method is provided wherein improvements ofthe geometry of rotors to be inserted into screw-type compressors areconsidered already when generating the negative mold. First, to thiseffect, a master rotor reproducing the shape of the winding teeth isproduced by removing material from a master rotor blank. This masterrotor, however, will then still comprise regions which do not yet havethe geometry desired for the completed rotor and thus are in need ofcorrection. A corresponding corrective treatment could be performed onthe master rotor itself. According to the method according to theinstant invention, however, this corrective treatment is performed byremoval of material from the blank of the negative mold so that thenegative mold is provided with deepened portions which serve forgenerating the complementary correction regions. When the thus correctednegative mold is filled with suitable material for a rotor, the rotorwill be formed with elevated portions corresponding to said deepenedportions of the negative mold. These elevated portions improve theproperties of the rotor in accordance with the desired geometry.

Preferably, the blank of the negative mold is subjected to an insideturning treatment. By this turning treatment, mold regions are generatedwhich during manufacture of the positive rotor will form cylindricalouter surfaces extending continuously over the complete length of therotor. This continuity safeguards a reliable sealing between the primaryrotor and the secondary rotor.

Preferred embodiments of the invention will be described in greaterdetail hereunder with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a rotor produced by the method of theinvention, which together with an appertaining second rotor forms arotor pair and is arranged in a compressor housing,

FIG. 2 is a view similar to FIG. 1 of a pair of rotors according to thestate of the art,

FIG. 3 is a plan view, partly broken away according to line III--III inFIG. 2, of the pair of rotors of FIG. 2,

FIGS. 4a) to 4c) show an enlarged detail of FIG. 2, with the rotorsarranged in different rotational positions,

FIGS. 5a) to 5c) are sectional schematic views of the constellationsaccording to FIGS. 4a) to 4c),

FIGS. 6a) to 6c) show enlarged details of FIG. 1, with the rotorsarranged in different rotational positions,

FIG. 7 is a view of a blow-hole delimited by the rotors and the housingshown in FIG. 1,

FIG. 8 is a view of a blow-hole delimited by the rotors and the housingshown in FIG. 2,

FIG. 9 is a sectional view of the pair of rotors according to FIG. 1 ina further engagement position of the rotors,

FIG. 10 shows an enlarged detail of FIG. 9,

FIG. 11 is a view of the master rotor for producing one of the tworotors according to FIG. 1,

FIG. 12 is a view of a blank of a negative mold having a geometry takenfrom the master rotor according to FIG. 11,

FIG. 13 is a view of a negative mold produced by an inside turningtreatment of the negative mold in FIG. 12, and

FIG. 14 is a view of a rotor produced by use of the negative mold ofFIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a view of a screw-type compressor 16 comprising a rotor 10produced according to the invention, provided as a primary rotor andbeing supported in a common housing 14 with a further rotor 12 providedas a secondary rotor. The two rotors 10,12 mesh with each other withinhousing 14 so that air is conveyed in axial direction and is compressed.The primary rotor 10 has five teeth 18 formed thereon, being distributedat equal distances over its periphery and wound at an angle of about240° along the length of primary rotor 10. The secondary rotor 12meshing with primary rotor 10 has six teeth 20 formed thereon which arewound at an angle of about 180° along the length of secondary rotor 12.

Within housing 14, the two rotors 10,12 are surrounded by a first andresp. a second housing portion 22,24 in such a manner that tooth flanks34,36 of the teeth 18 of primary rotor 10 and tooth flanks of the teeth20 of secondary rotor 12 in combination with said first and resp. asecond housing portions 22,24 define displacement chambers 26a to 26h.In the region before the pressure outlet of screw-type compressor 16,tooth flanks 34,36 of respective teeth 18 of primary rotor 10 and toothflanks of respective teeth 20 of secondary rotor 12 define a dischargechamber 28 between them. Further, a suction chamber 30 is defined beforethe inlet.

The efficiency of the illustrated screw-type compressor 16 essentiallydepends on the tightness of the displacement chambers 26a to 26h, thedischarge chamber 28 and the suction chamber 30, with the sealingbehavior of the mutually meshing teeth 18,20 having a large influence onthe efficiency of screw-type compressor 16.

For illustration of the sealing and flow conditions, a screw-typecompressor 116 according to the state of the art is shown in FIGS. 2 to5 and 8. This conventional screw-type compressor 116 is different fromthe screw-type compressor 16 including the inventive rotor 10 byessential details of the rotor design. To allow an easier survey, thoseelements of screw-type compressor 116 which correspond to elements ofscrew-type compressor 16 are designated by reference numerals increasedby 100 over the numerals in FIG. 1.

The spatial configuration of the displacement chambers 126a to 126h isillustrated in FIG. 3. The displacement chambers 126a to 126h aredefined by the tooth flanks 134,136 of the teeth 118,120 of primaryrotor 110 and resp. of secondary rotor 112 in combination with therespective housing portion 122,124. The displacement chambers 126a to126h follow a helically winding course and partially extend over thecomplete length of rotors 110,112. Since, during operation, the rotors110,112 rotate in opposite senses, the volume of the individual chamberswill change permanently, while the tooth flanks 134,136 in cyclicalchange will successively define displacement chambers 126a to 126h,discharge chambers 128 and suction chambers 130. Each successive time,respectively two displacement chambers will be united to form adischarge chamber, and, after discharge of the pressurized gas, twodischarge chambers will open again and form a suction chamber.Thereafter, the tooth flanks will define two separated displacementchambers.

If, for instance, the two rotors 110,112 in FIG. 2 are turned by a fewangular degrees in the rotational senses indicated by arrows A and B,the displacement chambers 126a to 126h unite to form a dischargechamber. Simultaneously, the volume of the existing discharge chamber128 is reduced so that the gas enclosed in discharge chamber 128 isexhausted at an increased pressure. At the same time, the volume of thesuction chamber 130 which extends up to the suction side of thescrew-type compressor 116, is increased. Thus, gas to be compressed issucked in.

The primary rotor 110 shown in FIG. 2 has been produced by machining andthus comprises rounded deepened portions 132 in each of its tooth-rootregions. Said rounded deepened portions 132 are required for allowingthe tooth flanks 134,136 to be treated by use of suitable tools. Tocause the primary rotor 110 and the secondary rotor 112 to roll againsteach other with a sealing effect while the rotors are meshing with eachother, the secondary rotor 112 has each of its teeth 120 provided with arounded edge 138 which engages the respective rounded deepened portion132 of primary rotor 110 while the two rotors 110,112 are meshing.

The rounded deepened portions 132 of the primary rotor 110 are locatedwithin the rolling circle 140 of primary rotor 110. Accordingly, theedges 138 of secondary rotor 112 are located outside the rolling circle142 of secondary rotor 112.

The flow conditions generated in the engagement region (detail IV inFIG. 2) during the meshing of the two rotors 110,112 are illustrated inFIGS. 4a to 4c, and the resultant sealing and flow conditions areillustrated in FIGS. 5a to 5c.

During the rotational movement of the two rotors 110,112, the teeth 118of primary rotor 110 with their peak line 144--delimiting the toothflanks 134,136--will travel along a cylindrical surface 146 of firsthousing portion 122. In a corresponding manner, the edges 138 of theteeth 120 of secondary rotor 112 will travel along a second cylindricalsurface 148 of second housing portion 124. The edges 138 and the peaklines 144 by their respective cylindrical surfaces 146,148 form sealingmembers. Thereby, the displacement chambers 126d, 126h and the dischargechamber 128 are separated from each other (FIGS. 4a,5a). However, whenthe rotors are turned still further, the condition shown in FIG. 4b willbe reached in which the peak line 144 of the tooth 118 of primary rotor110 does not form a sealing anymore with the cylindrical surface 146. Inthis rotational position, pressurized gas can flow back from dischargechamber 128 into displacement chamber 126. This backflow is representedby an arrow in FIG. 5b. Only when the rotors 110,112 have been turnedinto the position shown in FIG. 4c and the tooth 120 of secondary rotor112 has come into abutment with the tooth 118 of the primary rotor 110,the new discharge chamber 128 formed by the displacement chambers issealed tight.

In contrast to the state of the art, the primary rotor 10 according tothe invention (FIG. 1) comprises, instead of rounded portions, firstcylindrical outer surfaces 50 arranged on the rolling circle 40 of theprimary rotor 10. In correspondence thereto, the secondary rotor 12comprises second cylindrical outer surfaces 52 arranged on the rollingcircle 42 of the secondary rotor 12.

The flow and sealing conditions resulting from the use of the inventiverotor are shown in FIGS. 6a-6c. To obtain a sealing effect between therotors 10,12 and housing 14, the teeth 18 of primary rotor 10 incombination with a cylindrical surface 46 of housing 14 form firstsealings, and the second cylindrical outer surfaces 52 of the teeth 20of secondary rotor 12 in combination with a cylindrical surface 54 ofhousing 14 form second sealings. Since the secondary rotor 12, withotherwise unchanged rolling conditions, has a smaller diameter than thesecondary rotor 112 according to the state of the art, the housing edge56 defined by said cylindrical surfaces 46 and 54 is situated closer tothe point of the mutual engagement of the teeth 18,20 of the two rotors10,12 than is the case with the corresponding edge 158 in the state ofthe art (FIG. 6b). Thereby, the size of the blow-hole is reduced.

Further, the teeth 20 of secondary rotor 12 are brought into engagementwith the teeth 18 of primary rotor 10 without delay because thesecondary rotor 20 does not comprise rounded portions formed on theedges on the tips of its teeth (FIG. 6c). Thus, the blow-hole whichopens shortly in each cross section during the meshing process, isclosed again considerably earlier than in the state of the art.

A comparison between the sizes of the blow-holes can be made withreference to FIGS. 7 and 8. FIG. 7 shows the blow-hole 92 produced in arotor according to the invention. FIG. 8 shows the blow-hole 192 openingin rotors according to the state of the art.

Since the size of the blow-hole is decisive for the backflow lossesoccurring, such a comparison makes it clearly evident that the instantrotor configuration offers considerable improvements of the efficiencyof a screw-type compressor.

FIGS. 9 and 10 show the rotors of FIG. 1 in a further turning position.It is clearly visible that even during the mutual engagement of the tworotors the first and second cylindrical outer surfaces 50,52 form areliable sealing. Both cylindrical outer surfaces 50,52 extendcontinuously so that the gap which during rolling is formed therebetweenin successive sections has a consistent width. The continuouscylindrical outer surface 50 in combination with the tooth flank 36defines a helical line 56 which presents a sharp edge, effecting areduction of flow losses.

FIGS. 11 to 14 are illustrative of the method for producing a negativemold for the rotor shown in FIG. 1 and of the production of a rotorshown using such a negative mold.

FIG. 11 shows a master rotor 200 which, corresponding to the primaryrotor to be produced therewith, comprises five helically winding teeth218. The tooth flanks 234,236 have a contour which, in the regionbetween a peak line 244 formed by the tooth flanks 234,236 and therolling circle 240 of master rotor 200, is the same as the contour ofthe teeth 10 of the inventive primary rotor 10 and the primary rotor 110according to the state of the art. Since the master rotor 200, as is thecase for the primary rotor 110 according to the state of the art, isproduced from a master rotor blank by machining, it comprises roundeddeepened portions 232 below the rolling circle 240.

The thus produced master rotor is first placed into a mold box, and thecavity between the mold box and the master rotor is filled with asuitable mold material which is then cured. In its cured condition, themold material forms the negative mold blank 260.

After the curing of the mold material, the master rotors is taken out ofthe negative mold blank 260 so that a cavity 262 remains. The contour264 of cavity 262 comprises helically winding deepened portions 266whose geometry is complementary to the geometry of the teeth of aprimary rotor. Arranged between the deepened portions 266 are helicallywinding protruding regions 268.

For further treatment, the negative mold blank 260 is clamped into aturning lathe, in which raised molded edges 270 of the protrudingregions 268 are removed by an inside turning treatment. By thistreatment, there is obtained a negative mold 280 comprisinghollow-shaped, helically winding, continuous cylindrical surfaces 282.

By filling the negative mold shown in FIG. 13 with a suitable rotormaterial and by curing the material, the primary rotor 290 of FIG. 14 isproduced, its geometry coinciding with that of the primary rotor 10 ofFIG. 1.

Although a preferred embodiment of the invention has been specificallyillustrated and described herein, it is to be understood that minorvariations may be made in the apparatus without departing from thespirit and scope of the invention, as defined the appended claims.

I claim:
 1. A method for the production of rotors for screw-typecompressors using a master mold technique, comprising the followingsteps:a) producing a master rotor representing the shape of the windingrotor teeth, by removal of material from a master rotor blank, b)producing a negative mold blank by taking up the shape of the masterrotor, c) producing a negative mold by removal of material from thenegative mold blank to create deepened portions provided to generatecorrection regions, d) taking up the contour of the negative mold byfilling the negative mold with loose rotor material and curing saidmaterial, and e) separating the rotor from the mold.
 2. The methodaccording to claim 1, characterized in that rounded portions in thetooth-root regions generated during production of the master rotor, arecorrected by the removal of protrusions in the negative mold blank whichhave been generated by said rounded portions.
 3. The method according toclaim 2, characterized in that corrections of the profile of thenegative mold blank are performed by an inside turning treatmentperformed on the negative mold blank.
 4. A method for the production ofa negative mold for a rotor to be inserted into a screw-type compressorand having a plurality of helically winding teeth formed thereon,comprising the following steps:a) producing a master rotor representingthe shape of the winding rotor teeth, by removal of material from amaster rotor blank, b) producing a negative mold blank by taking up theshape of the master rotor, c) removal of material from the negative moldblank to create deepened portions provided to generate correctionregions.